Well, "best" is a bit much because lots of this is personal preference.

Anyways, I play in C Standard most of the time. Occasionally I will lower it down to B Standard, but I haven't done that for awhile.

I use D'Addario strings. I use the following diameters: .060, .046, .034, .022, .017, .012.

The 1st to 3rd strings are plain steel and the rest are nickel wound.

Compared to your average guitar in standard tuning this is noticeably more difficult to play. However, I selected these strings because they are on the light side for that tuning. It's not like spaghetti, but I do find it nice for bends. If you put these strings on a 24.75" guitar you may find the 6th string a bit too loose. I have the same set on both a 25.5" and a 24.75" and I like them both personally. You can only get this set from buying singles too, so that can be a pain in the ass when your store doesn't have .060s in stock.

EDIT: If you're going the singles route, make sure you stock up on strings so one doesn't break on you and your store doesn't carry the strings you need at the time. It's good to have at least two complete sets stored away along with extra strings. If they don't have your 6th string, but they have three of your 2nd string you might want to buy the three you can use because they might not be there when you go back.

Also, if you are worried about the strings rusting or something while you store them, just put each set in a plastic bag and knot it closed. Problem solved.

As far as 25.5 scale is concerned, I would say a set of 12's. Like D'Addario's 012-052 (or 011-052 if you like the high E string loose). I use 013-056, changing between C and B standard. C feels great and B is as functional as normal scale guitar can be.